1. Field of the Invention
This invention relates to semiconductor bonding tools for microwelding electronic interconnects, primarily in the microelectronics field and, more specifically to the bonding of wires and leads to conductor surfaces, such as bonding pads of semiconductor devices and associated structures.
2. Brief Description of the Prior Art
In the field of semiconductor technology, it is often necessary to interconnect electrically conductive areas. A common such interconnect involves connections between the bond pads of a semiconductor chip and the leads extending from the interior to the exterior of the package housing the semiconductor chip. Such interconnections are generally provided by bonding thin wires, tapes, film conductors and the like, generally of gold, aluminum, copper or various metal alloys. In the case of wires, such wires are generally of the order of one mil (0.001 inch) and are bonded or microwelded between the two points to be interconnected. Such bonding takes place in conjunction with well known wire bonding equipment.
The wire bonding equipment for providing the microwelds described above generally utilizes thermosonic, thermocompression or ultrasonic welding techniques.
Thermosonic bonding methods require bringing the wire, ribbon or TAB film into engagement with the conductive surface to which it to be welded and using a tool tip to force the surfaces to be welded (i.e., wire and bond pad) into close proximity in the contact area. Simultaneous heat, pressure and ultrasonic energy are applied to the metal surfaces beneath the tool tip to penetrate surface oxide layers and cause the molecules of the metals to interdiffuse to form the microweld or bond.
Thermocompression bonding is similar to thermosonic bonding except that the microweld or bond is formed using only heat and pressure. Ultrasonic bonding is also similar to thermosonic bonding except that the weld is formed by applying only ultrasonic energy. In each case, the tool tip is utilized.
Prior art bond tool tips have been provided utilizing many different materials and with many different geometries. For example, such prior art tool tips have included a circular, V-shaped, square or rectangular, etc. bond foot which is concave, convex, flat or comprised of a series of parallel or non-parallel grooves or raised surfaces to promote bonding or microwelding at conductor interconnect points. Prior art bonding tools have been formed of tungsten carbide, titanium carbide, steel alloys, tungsten carbide with an osmium tipped bonding foot and tungsten carbide with a ceramic tip bonding foot. The bonding tip or foot experiences a great deal of heat, abrasion and stress during the bond cycle. This promotes a variety of problems including material buildup or load up on the bonding foot, tip abrasion wear and degradation of tip or bonding foot geometries causing lost productivity, low quality bond formation and frequent tool changes.
The carbide material tools are a cemented matrix that consists of 6 to 10% binder. The metals used in the wires or TAB films adhere to this binder system and quickly load up. This will not only plug feed holes, thereby necessitating tool change, but, in addition, load up in the front radius or back radius leads to heel cracks. These heel cracks are a well documented cause for semiconductor device failure. The osmium tip tools, steel alloys and ceramic tip tools do not contain binder systems, but still have an affinity for the metals used in bonding and will also load up.
Furthermore, the current materials used for bond wedges, due to the binder and material characteristics thereof, will wear or abrade. The binder breaks out and the osmium and ceramic wear away. This necessitates frequent tool changes and subsequent lost productivity due to machine downtime. Since the face of the tools is abrading, the shape of the tool degrades over its lifetime. This alters the bond foot geometry in intimate contact with the wire or film and conductor surface and will cause inconsistent bond quality.
The abrasion and tool wear also changes the surface texture of standard tools during its bond life. This change in surface texture will affect how efficiently the ultrasonic energy is coupled to the bond wire, ribbon or film. Since the ultrasonic energy is fixed for a process, an increase in tool slippage due to surface changes will lead to inconsistent bond quality during the life of the tool. The increased hardness that the copper films, which are generally used in TAB bonding, exhibit causes current tools to be very susceptible to this phenomenon.